We propose to analyze the structure of the viral integration sites on the chromosomes of lambda and E. coli and to identify those features which are relevant to the integration-excision process. Our progress thus far has consisted of establishing a tentative sequence (less than 5% of the positions have ambiguous nucleotide assignments) for the four lambda att structures, POP', POB', BOP', and BOB'. Approximately 200 base pairs of sequence have been determined for each structure. We propose to eliminate the ambiguities in the sequence and to extend our studies to include appropriately chosen deletions, point mutants, and secondary att sites. A comparison of these structures with the biology which has already been worked out for many of them should help us to identify the essential structural features of the integration and excision pathways. The precise location of the crossover point for the site-specific recombination will be determined by analyzing the results of an in vivo int-dependent recombination in which one of the parents is substituted with BUdR. We also propose to analyze the relevant features of the att structure by chemically generating classes of point mutants and deletions which cannot be obtained in vivo. The level and properties of the residual functions of these mutants will be compared with the induced alterations in their structures. These studies will be supplemented by an analysis of the interaction of int and xis proteins with the att structures.